1. Field of the Invention
This invention relates to an aligning apparatus and method, and more particularly to an apparatus such as a semiconductor circuit pattern exposure apparatus (a so-called mask aligner) which effects precise alignment.
2. Description of the Prior Art
Generally, semiconductor elements, IC, etc. are manufactured by repeating the steps of printing a complicated circuit pattern on a substrate of Si, GaAs or the like and applying a chemical or physical treatment thereto.
In this case, there arises the necessity of accurately aligning the pattern for the next step with the predetermined position of the substrate subjected to the preceding step of processing.
In recent years, making the patterns minute has been pursued to provide a higher production speed and higher integration and alignment accuracy as high as the order of 0.1 .mu.m has been required.
Heretofore, such alignment has been carried out by the operator through his visual observation, but in recent years, attention has become directed to automation of such work, namely, autoalignment. For example, the scanning type light detecting apparatus described in U.S. Pat. No. 4,165,149 assigned to the same assignee as the assignee of the present invention in such that a laser light is imaged in the form of a spot or a slit on a body and scans the body at a uniform speed, whereby the scattered light from a mark provided for alignment is detected. By the advent of the laser light scanning method, it has become possible to extract a signal of good S/N ratio even from a semiconductor wafer on which a circuit pattern has been printed and which has a complicated minute cross-sectional shape and thus, the autoalignment technique has rapidly become popular.
The laser light scanning system, as seen, for example, in the photoelectric detecting apparatus of U.S. Pat. No. 4,251,129 assigned to the same assignee as the assignee of the present invention, is applicable not only to the contact method in which the spacing between a mask and a wafer is zero or the proximity method in which the spacing between the mask and the wafer is slight, but also to the projection method using a lens or a mirror.
Now, as a factor for improving the accuracy of autoalignment in the field concerned, solution of the problem of interference of laser light has become a great task. That is, the edge signal output of the alignment mark of the mask fluctuates and stable alignment is difficult to achieve. This is shown in FIG. 1 of the accompanying drawings. In FIG. 1, reference numeral 1 designates a mask formed with a pattern to be printed, reference numeral 2 denotes a wafer, and reference numeral 3 designates an incident light. There are two kinds of light scattered by the edge of the alignment mark of the mask 1.
One is the light directly scattered and returning from the edge of the alignment mark, as indicated by 4 in FIG. 1, and the other is the light reflected by and returning from the wafer 2. There are two kinds of light reflected by and returning from the wafer 2. One is the light scattered by the edge of the alignment mark of the mask 1 and reflected by the wafer 2, as indicated by 5, and the other is the light passing the edge of the alignment mark of the mask 1, reflected by the wafer 2 and thereafter scattered by the edge of the alignment mark, as indicated by 6. The wafer usually has a high reflection factor and therefore, the interference between the light 4 directly scattered from the edge of the alignment mark of the mask 1 and the light 5 or 6 reflected by the wafer 2 becomes great and, during alignment, due to a minute change between the mask and the wafer, the alignment signal output of the photomask fluctuates markedly and becomes unstable. This effect has adversely affected the alignment accuracy.
In the foregoing, it has been described that the edge signal output of the alignment mark of the mask 1 becomes unstable by the interference resulting from the presence of the wafer 2, and besides this, it is theoretically conceivable that the edge signal output of the alignment mark of the wafer 2 becomes unstable by the interference resulting from the presence of the mask.
That is, the light directly scattered from the edge of the alignment mark of the wafer and the light reflected by the underside of the mask, again returning to the wafer and scattered from the edge of the alignment mark of the wafer interfere with each other.
However, since the mask is usually formed of a material such as glass which has a low reflection factor, the interference thereof is weak actually and thus, the edge signal output of the alignment mark of the wafer is stable.
The influence of the interference on the edge signal output of the alignment mark of the mask is remarkable in proximity printing wherein the gap between the mask and the wafer is slight, but this is still a problem again in the case of the projection printing using a lens or mirror optical system because the interference distance of laser light is long.